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<p>写【高并发专题】有一段时间了，一些读者朋友留言说，并发编程很难，学习了很多的知识，但是在实际工作中却无从下手。对于一个线上产生的并发问题，又不知产生这个问题的原因究竟是什么。对于并发编程，感觉上似乎是掌握了，但是真正用起来却不是那么回事！</p> <p>其实，造成这种现象的本质原因就是没有透彻的理解并发编程的精髓，而学好并发编程的关键是需要弄懂三个核心问题：<font color="00FF00"><strong>分工、同步和互斥</strong>。</font></p> <h2 id="分工"><a href="#分工" class="header-anchor">#</a> 分工</h2> <blockquote><p>比较官方的解释为：分工就是将一个比较大的任务，拆分成多个大小合适的任务，交给合适的线程去完成，强调的是性能。</p></blockquote> <p>如果你还不能够理解什么是分工，这里，我们可以做一个假设。假设你是一个XXX上市公司的CEO，你的工作是如何管理好你的公司。但是，就如何管理好公司而言，涉及到的任务就比较多了，我们可以将其看做一个很大的任务，这个很大的任务，细看的话可以包括：人员招聘和管理、产品设计和开发、运营和推广、公司税务等等。那细化后这么多的任务交给你一个人去做，想必你一定是崩溃的。即使你能够挺住，估计你一个人把这所有的任务完成，那黄花菜也就凉了！到时，估计你就会偷偷的躲在角落里唱“凉凉”了。。。</p> <p>所以，如果你真的想管理好你的公司，你就需要将这些任务分解，分工细化，将人员招聘和管理的任务交给人力资源部门去完成，将产品的设计交给设计部门去完成，将产品的开发交给开发部门去完成，将运营和推广交给运营和市场部门去完成，将公司税务交给财务部门去完成。此时，你的任务就是及时了解各个部门的工作情况，统筹并协调各部门的工作，并思考如何规划公司的未来。</p> <p>其实，这里你将管理公司的任务拆解、细化分工之后，你会发现，其实各部门之间的工作是并行执行的。比如：人力资源部门在管理员工的绩效考核时，同时产品设计和开发部门正在设计和开发公司的产品，与此同时，公司的运营正在和设计与开发沟通如何更好的完善公司的产品，而推广部门正在加大力度宣传和推广公司的产品。而财务部门正在统计和计算公司的各种财务报表等。一切都是那么的有条不紊！</p> <p><img alt="" data-src="https://img-blog.csdnimg.cn/20200322144449355.jpg" loading="lazy" class="lazy"></p> <p>所以，安排合适的人去做合适的事情，在实际工作中是非常重要的。这映射到并发编程领域也是同样的道理。如果将所有的任务交给一个线程执行，就好比将公司的所有事情交给你一个人去做一样。等到把事情做完了，黄花菜也凉了。所以，在并发编程中，我们同样需要将任务进行拆解，分工给合适的线程去完成。</p> <p><img alt="" data-src="https://img-blog.csdnimg.cn/20200322144522304.jpg" loading="lazy" class="lazy"></p> <p>在并发编程领域，还需要注意一个问题就是：<strong>分工给合适的线程去做。</strong> 也就是说，<strong>应该主线程执行的任务不要交给子线程去做，否则，是解决不了问题的</strong>。这就好比一家公司的CEO将如何规划公司的未来交给一个产品开发人员去做一样，这不仅不能规划好公司的未来，甚至会与公司的价值观背道而驰。</p> <p><strong>在JavaSDK中的：Executor、Fork/Join和Future都是实现分工的一种方式。</strong></p> <h2 id="同步"><a href="#同步" class="header-anchor">#</a> 同步</h2> <blockquote><p>在并发编程中的同步，主要指的是一个线程执行完任务后，如何通知其他的线程继续执行，强调的是性能。</p></blockquote> <p>将任务拆分，并且合理的分工给了每个人，接下来就是如何同步每个人的任务了。</p> <p>假设小明是一名前端开发人员，他渲染页面的数据需要等待小刚的接口完成，而小刚写接口又需要等待小李的服务开发完成。也就是说，<strong>任务之间是存在依赖关系的，前面的任务完成后，才能进行后面的任务。</strong></p> <p>对于实际工作中，这种任务的同步，大多数靠的是人与人之间的沟通，小李的服务写完了，告诉小刚，小刚则马上进行接口开发，等小刚的接口开发完成后，又告诉了小明，小明马上调用接口将返回的数据渲染在页面上。</p> <p><img alt="" data-src="https://img-blog.csdnimg.cn/20200322144547517.jpg" loading="lazy" class="lazy"></p> <p>这种同步机制映射到并发编程领域，就是<strong>一个线程的任务执行完毕之后，通知其他的后续线程执行任务。</strong></p> <p>对于这种线程之间的同步，我们可以使用下面的 <strong>if</strong> 伪代码来表示。</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">if</span><span class="token punctuation">(</span>前面的任务完成<span class="token punctuation">)</span><span class="token punctuation">{</span>
    执行当前任务
<span class="token punctuation">}</span><span class="token keyword">else</span><span class="token punctuation">{</span>
    继续等待前面任务的执行
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>如果为了更能够及时的判断出前面的任务是否已经完成，我们也可以使用 <strong>while</strong> 伪代码来表示。</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">while</span><span class="token punctuation">(</span>前面的任务未完成<span class="token punctuation">)</span><span class="token punctuation">{</span>
    继续等待前面任务的执行
<span class="token punctuation">}</span>
执行当前任务
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上述伪代码表示的意义是相同的：<strong>当线程执行的条件不满足时，线程需要继续等待，一旦条件满足，就需要唤醒等待的线程继续执行。</strong></p> <p>在并发编程领域，一个典型的场景就是生产者-消费者模型。当队列满时，生产者线程需要等待，队列不满时，需要唤醒生产者线程；当队列为空时，消费者线程需要等待，队列不空时，需要唤醒消费者。我们可以使用下面的伪代码来表示生产者-消费者模型。</p> <ul><li>生产者</li></ul> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">while</span><span class="token punctuation">(</span>队列已满<span class="token punctuation">)</span><span class="token punctuation">{</span>
    生产者线程等待
<span class="token punctuation">}</span>
唤醒生产者
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><ul><li>消费者</li></ul> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">while</span><span class="token punctuation">(</span>队列为空<span class="token punctuation">)</span><span class="token punctuation">{</span>
    消费者等待
<span class="token punctuation">}</span>
唤醒消费者
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p><strong>在Java的SDK中，提供了一些实现线程之间同步的工具类，比如说：CountDownLatch、 CyclicBarrier 等。</strong></p> <h2 id="互斥"><a href="#互斥" class="header-anchor">#</a> 互斥</h2> <blockquote><p>同一时刻，只允许一个线程访问共享变量，强调的是线程执行任务的正确性。</p></blockquote> <p>在并发编程领域，分工和同步强调的是执行任务的性能，而线程之间的互斥则强调的是线程执行任务的正确性，也就是<strong>线程的安全问题</strong>。如果多个线程同时访问同一个共享变量，则可能会发生意想不到的后果，而这种意想不到的后果主要是由线程的<strong>可见性、原子性和有序性</strong>问题产生的。而解决可见性、原子性和有序性问题的核心，就是互斥。</p> <p>关于互斥，我们可以用现实中的一个场景来描述：多个岔路口的车辆需要汇入一条道路中，而这条道路一次只能允许通过一辆车，此时，车辆就需要排队依次进入路口。</p> <p><strong>Java中提供的synchronized、Lock、ThreadLocal、final关键字等都可以解决互斥的问题。</strong></p> <p>例如，我们以synchronized为例来说明如何进行线程间的互斥，伪代码如下所示。</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">//修饰方法</span>
<span class="token keyword">public</span> <span class="token keyword">synchronized</span> <span class="token keyword">void</span> <span class="token function">xxx</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    
<span class="token punctuation">}</span>
<span class="token comment">//修饰代码块</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">xxx</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">synchronized</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">{</span>
        
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token comment">//修饰代码块</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">xxx</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">synchronized</span><span class="token punctuation">(</span>XXX<span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token comment">//修饰静态方法</span>
<span class="token keyword">public</span> <span class="token keyword">synchronized</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">xxx</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br></div></div><h2 id="总结"><a href="#总结" class="header-anchor">#</a> 总结</h2> <p>并发编程旨在最大限度的利用计算机的资源，提高程序执行的性能，这需要线程之间的分工和同步来实现，在保证性能的同时，又需要保证线程的安全，这就又需要保证线程之间的互斥性。而并发编程的难点问题，往往又是由可见性、原子性和有序性问题导致的。所以，我们在学习并发编程时，一定要先弄懂线程之间的分工、同步和互斥。</p> <p>最后，附上并发编程需要掌握的核心技能知识图，祝大家在学习并发编程时，少走弯路。</p> <p><img alt="" data-src="https://img-blog.csdnimg.cn/20200322144644983.jpg" loading="lazy" class="lazy"></p> <h2 id="写在最后"><a href="#写在最后" class="header-anchor">#</a> 写在最后</h2> <blockquote><p>如果觉得文章对你有点帮助，请微信搜索并关注「 <strong>冰河技术</strong> 」微信公众号，跟冰河学习高并发编程技术。</p></blockquote> <p>最后，附上并发编程需要掌握的核心技能知识图，祝大家在学习并发编程时，少走弯路。</p> <p><img alt="" data-src="https://img-blog.csdnimg.cn/20200322144644983.jpg?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2wxMDI4Mzg2ODA0,size_16,color_FFFFFF,t_70#pic_center" loading="lazy" class="lazy"></p> <blockquote><p>如果你觉得冰河写的还不错，请微信搜索并关注「 <strong>冰河技术</strong> 」微信公众号，跟冰河学习高并发、分布式、微服务、大数据、互联网和云原生技术，「 <strong>冰河技术</strong> 」微信公众号更新了大量技术专题，每一篇技术文章干货满满！不少读者已经通过阅读「 <strong>冰河技术</strong> 」微信公众号文章，吊打面试官，成功跳槽到大厂；也有不少读者实现了技术上的飞跃，成为公司的技术骨干！如果你也想像他们一样提升自己的能力，实现技术能力的飞跃，进大厂，升职加薪，那就关注「 <strong>冰河技术</strong> 」微信公众号吧，每天更新超硬核技术干货，让你对如何提升技术能力不再迷茫！</p></blockquote> <p><img alt="" data-src="https://img-blog.csdnimg.cn/20200906013715889.png" loading="lazy" class="lazy"></p></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/binghe001/BingheGuide/edit/master/docs/md/concurrent/basics/2020-03-30-003-要想学好并发编程，关键是要理解这三个核心问题.md" target="_blank" rel="noopener noreferrer">在 GitHub 上编辑此页</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <div class="last-updated"><span class="prefix">上次更新: </span> <span class="time">2022/5/23</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev">
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